The present invention relates to doping of a substrate, particularly of semiconductor material, and has for an object to provide a doping method and apparatus which make it possible to achieve high concentrations of doping impurities over a small, but well determined, thickness, from the surface of such substrate.
Numerous semiconductor devices are prepared by doping a substrate with acceptor or donor impurities at a concentration close to the solubility limit, over a very small superficial thickness but in a precisely defined concentration. Examples of such devices are certain types of integrated circuits, some types of nuclear radiation detectors and photovoltaic cells, in which the surface conductivity must be as high as possible to reduce the conversion yield losses. Conventional surface doping techniques by thermal diffusion or ionic implantation having shortcomings when attempts are made to attain this result, in particular for producing photovoltaic solar cells. The first above-named technique is not applicable to polycristalline substrates for the available impurities concentrate at grain boundaries. Direct ionic implantation leads to high costs for the equipment is complex and the ionic currents obtained have in general too low a density for processing large surfaces in acceptable periods of time.
It is an object of the present invention to provide an improved doping process, particularly for producing PN junctions and ohmic or rectifying contacts, by ionic bombardment of a semiconductor substrate, allowing high and homogenous concentrations to be obtained in an economical way.
According to a first aspect of the invention, there is provided a process for doping a semiconductor substrate comprising the steps of maintaining an electric discharge in an atmosphere containing a gaseous compound of a dopant for creating ions of said dopants; extracting and accelerating a beam of said ions from said atmosphere towards said substrate under a voltage selected for achieving a predetermined doping profile; and circulating said substrate transversely to said beam. It should be noted that this process, while related to conventional processes of ionic implantation in so far as the doping process properly speaking is concerned, is fundamentally different therefrom as regards the method of creating the ion beam which is accelerated towards the substrate: the ion currents which can be obtained are greater by several orders of magnitude than those possible with conventional ionic implantation.
Either one surface of the substrate, or both, can be treated, depending on the ultimate use of the processed article.
Among the applications of the process, there can be mentioned particularly the manufacture of photovoltaic cells by implantation of ions on a silicon substrate which will typically be polycrystalline, while however other forms of silicon may be used. The substrate can be in the form of a ribbon, which allows a particularly economical use thereof, or successive thin boards or plates.
In accordance with another aspect of the invention, there is provided an apparatus for doping a semiconductor substrate by ion implantation, comprising: a discharge chamber; means for maintaining an atmosphere containing a gaseous compound of a dopant in said chamber under sub-atmospheric pressure; at least a pair of electrodes one of which is formed with at least one ion extraction opening and separates said discharge chamber from an implantation chamber arranged for receiving the substrate; electric supply means for establishing an electric discharge in said chamber between said electrodes; ion accelerating electrode means located in proximity to the extraction electrode, between the extraction electrode and the substrate; and means for applying an acceleration voltage to the accelerating electrode means.
In a particular embodiment, the chamber which receives the substrate, and which forms an implantation chamber, is provided with means for introducing the substrate, for circulating it along a path across the ion beam and for extracting it therefrom. This chamber is provided with means for maintaining therein a vacuum which is generally between 10.sup.-2 and 10.sup.-4 mm of mercury. The ionic current densities will be in general between about 0.1 mA/cm.sup.2 and 1 mA/cm.sup.2, at least when a single extraction orifice is formed. It is however possible to use an output electrode formed with a plurality of extraction openings and then the overall current density can be increased. It is also possible to provide a plurality of discharge chambers each delivering a ion beam to the implantation chamber.
The discharge and acceleration voltages will generally respectively be a few kV and a few tens of kV.
The parts which constitute an ion source can be formed as a unit which is simple and low weight. That unit can be movably connected to the implantation chamber and provided with a scanning mechanism, thereby permitting to implant ions on large areas.
The technique which is used makes it possible to omit selection magnets and stabilization of the HV electric supply. As a consequence, it is possible either to use a low cost HV supply or to use a modulated HV supply for achieving inhomogeneous doping precisely for specific purposes.
The invention will be better understood from reading the description of a device forming a particular embodiment, for treating a ribbon of polycrystalline silicon.